SERVER-30249 made mongos block during startup until it receives the keys used to sign cluster times from the config server, because it won't gossip $clusterTime to clients without them. This is skipped if the max wire version of mongos's RSM for the CSRS is less than the OP_MSG wire version or if the CSRS RSM's min and max wire version are different.

It seems the second part of this check was to avoid waiting on keys when the CSRS was in FCV 3.4, but it consistently fails in master (not 4.0 or 3.6) so mongos skips waiting for keys. This means mongos is not guaranteed to gossip $clusterTime and provide causal consistency immediately after starting up, although it will as soon as it receives keys from the config sever, which normally happens quickly.